Polyurethane formulations find widespread use, in particular in the construction industry. They are for instance used as adhesives, such as for mounting doors and window frames, as mastics and sealants, such as for sealing and insulation purposes. The formulations may foam upon application or they may not foam. They may also initially foam and subsequently collapse, and thus not end up as foam. Foaming formulations may lead to a foamed end product, and this may have closed cells or open cells. The foam formed may also be made to collapse and end up as a high density layer. The formulations are typically produced and offered in pressurized containers, cans or vessels. For more sophisticated applications, the formulations may be pumped from containers or be pushed out of those containers by putting these under pressure by means of an external pressure gas.
A polyurethane (PUR or PU) is a polymer composed of a chain of organic units joined by the carbamate or urethane link. PU polymers are formed through step-growth polymerization, by reacting one or more monomers having at least two isocyanate functional groups with at least one other monomer having at least two isocyanate reactive groups, i.e. functional groups which are reactive towards the isocyanate function. The isocyanate (—N═C═O or “NCO”) functional group is highly reactive and is able to react with many other chemical functional groups. In order for a functional group to be reactive to an isocyanate functional group, the group typically has at least one hydrogen atom which is reactive to an isocyanate functional group. Most frequently compounds are used having at least two hydroxyl or alcohol groups. Where di-isocyanate molecules are reacting with other difunctional molecules, so-called “linear” polymers are formed. Where at least one of the isocyanates or one of the other molecules has three or more functional groups, the polymer structure is able to cross-link and form three-dimensional structures. The structures with a low degree of cross-linking lead to the more elastic products. For adhesives on the other hand, polymer structures with ultimately a high degree of cross-linking are preferred.
The reaction of an isocyanate monomer with a second reactant may be favoured by the presence of one or more catalysts. Suitable catalysts are amine compounds, typically tertiary amines, and organometallic compounds.
Also water is reactive towards the isocyanate function, and typically plays a role in the final curing of the polymer towards the formation of an ultimately rigid structure. The final “curing” of the polyurethane polymer, which may include further chain building as well as cross-linking, is often obtained at least partly by reaction with water, such as with atmospheric moisture or with moisture present in the substrate onto which the PU formulation is applied. An isocyanate functional group may react with water followed by liberating gaseous CO2 to form a primary amine, a functional group which is able to react at least once with more isocyanate functional groups. This mechanism thus also may lead to cross-linking in the polymer. The liberated CO2 may act as a (secondary) foaming agent. Thanks to this mechanism a polyurethane polymer structure with residual isocyanate functionality is able to cure or harden under the influence of atmospheric moisture, and depending on mixture viscosity at the same time may even foam further.
In a one component system, a polyurethane foam pressure container may for instance be prepared by introducing a mixture of towards isocyanate polyfunctional reactive compounds, typically higher molecular weight polyols and more typically polyether polyols, together with a stoichiometric excess of polyfunctional isocyanates into the can, and by giving the mixture sufficient time and shaking to mix the can content well, and to have it react until all towards isocyanate reactive functions are substantially reacted away and substantially only free isocyanate functions remain available. The so-formed viscous liquid mixture, regardless whether it is in the can or in a reaction vessel, is usually called a “prepolymer”. Propellant gasses may be added, optionally together with the reactants, to provide a pressure in the can, if desired. When using liquefied gasses such as LPG-type components or dimethyl ether (DME) as the propellants, these gasses may also act as a solvent for the other components in the mixture.
The application of the polyurethane then consists of releasing the viscous prepolymer mixture from the pressurized can and let it cure, in a one component foam formulation (OCF) by the reaction with atmospheric moisture and optionally also with water from a wet or moist substrate. The propellant gasses, together with the liberating CO2, may provide a foaming effect upon the expansion of the prepolymer to atmospheric pressure. The addition to the mixture of a stabilizer may help because such a component may act as a nucleator for starting the formation of gas bubbles, into which the propellants and the CO2 may then migrate. The curing reactions further increase the viscosity of the reacting mixture, which eventually sets as a solid polyurethane. The rate of curing is usually much faster than the rate at which the gasses are able to escape from the solidifying mixture, such that usually a solid foam structure is obtained.
In a two component system, the prepolymer with its remaining free isocyanate functionality is at the moment of application mixed with a second component containing a hardener, i.e. a polyfunctional isocyanate reactive compound, typically a low molecular weight polyol, preferably a component having primary alcohol functions, which is introduced from a second container. The result is a very fast curing mixture forming a high density product with high mechanical properties, very suitable for mounting doors and window frames. Optionally some water may be added to the polyol, which causes CO2 to form, and in which case propellants may not be essential in order to obtain a foam.
The stoichiometric ratio of the isocyanate functionality relative to the isocyanate reactive functionality in the mixture is usually referred to as the “index” of the prepolymer, i.e. the molar ratio, often being expressed as a percentage and possibly even without mentioning the percentage indicator, of all isocyanate functional groups present in the prepolymer mixture relative to the total number of functions reactive towards an isocyanate functional group, and this prior to the occurrence of any condensation reaction. The isocyanate index for a formulation is thus a measure of the excess isocyanate functionality used relative to the theoretical equivalent amount required.
The result of the reaction as described for forming the prepolymer is typically a mixture of unreacted monomers and chain-growth condensation products having various chain lengths, typically following a Schultz-Flory distribution. With the isocyanates having been in stoichiometric excess, practically all left over monomers are polyisocyanates. The “index” of the mixture strongly determines the shape of the molecular weight distribution curve, which will heavily affect the product viscosity, and the residual amount of free monomer. A lower index leads to a broader distribution and lower residual monomer content, but at the same time also to a higher viscosity reaction product. A higher index leads to a lower viscosity reaction product, but with a higher residual monomer content. The mixture which is formed in the pressure container should end up having a viscosity which is sufficiently low to allow it to be dispensed from the can. Low index prepolymers are therefore more difficult to work into suitable formulations for polyurethane foam canisters.
Well known polyisocyanates are isophorone diisocyanate (IPDI), toluene diisocyanate (TDI), naphthalene-1,5-diisocyanate (NDI), hexane-1,6-diisocyanate (HDI) and in particular the methyl diphenyl diisocyanates (MDI), more preferably the 2,2′-, the 2,4′- and the 4,4′-isocyanate isomers thereof.
The polyisocyanates are commercially available in a variety of forms. Due to their method of production, most of the commercial products are mixtures of diisocyanates with isocyanates having 3 and more isocyanate functional groups per molecule, i.e. with a higher functionality. The average functionality of the mixture of polyisocyanates used in the formulation is therefore one element which may influence the degree of cross-linking in the ultimate polyurethane polymer structure.
A very common commercial form of MDI is so-called crude MDI or polymeric MDI (pMDI). It is a mixture of polyfunctional isocyanate monomers having a different number of phenyl isocyanate functions. The diisocyanate monomer with its two phenyl cores typically makes up close to half of the mixture, the rest being 3-core, 4-core, 5-core, and higher monomers, each phenyl core typically offering one isocyanate functionality. The average functionality of pMDI is typically about 2.7.
Also the polyfunctional towards isocyanate reactive compound may have more than two reactive functions per molecule, or may be a mixture of compounds having a different number of functionalities. The average functionality of the polyfunctional towards isocyanate reactive compound or of the mixture of polyfunctional towards isocyanate reactive compounds is therefore another important element which may influence the degree of cross-linking obtainable in the ultimate polyurethane polymer structure.
The more popular polyols used in polyurethanes are polyether polyols, usually made by the reaction of epoxides (oxiranes) with an active hydrogen containing starter compound, and to a lesser degree also polyester polyols, which may be formed by the polycondensation of multifunctional carboxylic acids and hydroxyl compounds.
The nature of the towards isocyanate reactive compound, not only its average functionality, may also be important for the final properties of the polyurethane product formed. The polyfunctional towards isocyanate reactive compounds may for that purpose for instance be classified according to the functionality of the compound and to their molecular weight.
For a number of reasons, such as a better control of the final mixture obtained in the can after reaction, or of the foam properties, for obtaining a formulation giving a faster final cure and less heat generation when the reactants are mixed together in the pressure can or container, polyurethane formulations may make use of an externally synthesized prepolymer. These are prepared in a similar way as the prepolymer in the can described above, i.e. as a mixture of unreacted polyisocyanate monomers and polymers thereof obtained by step-growth condensation, following a Schultz-Flory distribution. The properties of these prepolymers may be influenced by their index and by the choice of the starting materials, such as the selection of the polyisocyanate and/or of the polyol, or by using mixtures of them.
Such prepolymers are available commercially. Popular examples are made by reacting monomeric MDI (mMDI) or the cheaper crude MDI with a small amount of diols and/or triols. These conventional commercial prepolymers typically still contain significant amounts of the diisocyanate monomer.
Recently, the presence of free diisocyanate monomers which remain as a residual component in the formulation at the moment of application of the PU composition, has become of concern. The diisocyanate monomer has a relatively low molecular weight and molecular size, and a relatively high vapour pressure as compared to most other formulation ingredients, including its higher functionality homologues. They are therefore relatively mobile, and are able to migrate from the formulation into its surroundings, such as the surrounding air, or into liquids or solids which are in contact with the formulation. Because of the high reactivity of the isocyanate functional group, these compounds may be harmful, due to their possible irritant, allergenic and/or toxic effects. This has therefore generated environmental, industrial health and toxicity concerns.
In view of this reactivity and mobility concern, formulators have already selected to preferably use methyl diphenyl diisocyanate (MDI) over other monomers having a lower molecular weight, such as IPDI, TDI or HDI. The use of polymeric MDI and/or prepolymers made thereof has further reduced the remaining presence of the 2-phenyl core diisocyanate monomer, including the three structurally different isomers, in the commercially offered product, which are with these starting materials the compounds of particular concern.
In certain countries, this concern has already lead to legislation which imposes enhanced safety measures, such as for increased ventilation, for the working area where the formulations are being applied, and which prescribes the use of toxicity labels and risk phrases (R40) on commercially offered packages with formulations containing the diisocyanate monomer at concentrations above a specified level. Recent EU legislation for instance, imposes the R40 risk phrase on packages containing formulations with a residual monomer MDI content of at least 1.0 wt %. Because the residue in used conventional PU foam containers may still contain as much as 15% residual free MDI monomer, these used containers may under many legislations have to be discarded as toxic waste.
Various methods have therefore been proposed for obtaining low residual levels of free diisocyanate monomers in the formulation, thereby reducing the concerns associated with using polyurethane formulations, and where possible, at least avoiding the R40 risk phrase.
WO 03/006521 A1 describes the use of asymmetric polyisocyanates in building the prepolymer. With asymmetric polyisocyanates, the different isocyanate functional groups in the monomer have a different reactivity. The lower reactivity functional groups will tend to survive as part of the prepolymer chain, and the higher reactivity monomer will tend to react away faster. This allows obtaining a prepolymer with lower monomer content for the same index, or a prepolymer having a lower viscosity for the same residual monomer content. The possibilities for generating suitable viscosity prepolymers having the desired low monomer content with this method remain however limited. Also the asymmetric polyisocyanates are rather difficult and costly to obtain. The prepolymers produced are also slower in the cross-linking reactions, which is another drawback.
WO 2007/115971 proposes to remove a major portion of the diisocyanate molecules from the pMDI mixture before forming the prepolymer. The document describes the preparation of a low monomer containing MDI oligomer mixture from technical grade MDI or from polymeric MDI (pMDI) by removing most of the MDI monomer by distillation. This separation is rather difficult and in order to avoid excessive temperatures, at which the molecules may degrade, makes use of sophisticated equipment such as a thin film, falling film or wiped film evaporator, operating under relatively deep vacuum. The amount of diisocyanate monomer to be removed is also significant. WO 2007/115971 further proposes to increase the molecular weight of the starting polyisocyanate mixture by increasing the average number of phenyl groups. This however leads to undesired byproducts, such as uretonimine, which needs to be removed, such as by extraction. This adds significant further complexity to the overall process.
Another proposal is described in several documents, including WO 00/04069, WO 2011/036018 and WO 01/014443, and exists in removing diisocyanate monomers from the prepolymer by distillation. Also this separation is difficult, and again a thin film, falling film or wiped film evaporator is proposed, usually under a deep vacuum.
These distillations require a complex additional step in the production process. Also, removing the lower molecular weight diisocyanate monomers increases the viscosity of the remaining stream significantly. WO 01/40340 proposes to add an inert solvent whose boiling point is slightly below that of the monomeric diisocyanate in order to facilitate the distillation. WO 02/079291 and WO 02/079292 propose the addition into the prepolymer mixture before distillation of an extra low viscosity and high boiling component which must be non-reactive towards isocyanate and hydroxyl groups, in order to alleviate this problem of viscosity increase. In order for the prepolymer remaining after distillation to have a sufficiently low viscosity, the upstream prepolymer reaction must be performed with a relatively high index, i.e. with a high excess of polyisocyanates, which means that the amount of residual monomer in the prepolymer mixture is typically still very high, possibly 50-60 wt %, or even as high as 80 wt %. The amount of monomer to be removed is thus very high, and this also makes it very difficult to obtain the low monomer levels desired. This extra step in the production process is therefore very complex and has a relatively low product yield. It is also a significant consumer of heat at a relatively high temperature level. Because of the complex equipment and the energy consumption, this additional step is expensive.
EP 1674492 proposes to use such monomer-poor pMDI, with a rest content of 0.2 wt % of monomeric MDI (mMDI), in the preparation of a prepolymer which is later used in a PU foam formulation. EP 1674492 discloses in the preparation of the polyol component the addition of an NCO prepolymer, named Komponente (ii), to which a small amount of diaceton alcohol is added as a chain regulator. The polyol component is subsequently reacted with the monomer-poor pMDI described above. The presence of mMDI in the preparation of EP 1674492 is so low that when all mMDI reacts twice with the diaceton alcohol, the concentration of this reaction product in the prepolymer composition is not higher than 0.441 wt %, and because of dilution not more than 0.331 wt % in the PU foam composition contained in the pressure container.
Several other documents also describe the use of monofunctional towards isocyanate reactive compounds in PU formulations.
EP 125008 describes the preparation of a polyurethane prepolymer by first reacting pure 4,4′-diphenyl methane diisocyanate with as polyol a linear polyester, to form an intermediate having an index of 1.83. In a second step, a small amount of 2-ethyl hexanol, equivalent to only 10% of the isocyanate functions remaining after the first reaction and representing less than 4.7% of all the isocyanate functional groups involved in forming the polyurethane prepolymer composition. The prepolymers of EP 125008 are formulated into a hot melt adhesive composition.
US 2010/0152381 is concerned with providing prepolymer systems having reduced monomeric isocyanate contents, and which may be useful in preparing articles, such as water-blown polyurethane foams. Monomeric isocyanate contents of no greater than about 10% by weight are considered as being reduced. The document proposes to prepare diluent components by reacting a polymethylene polyphenyl polyisocyanate (pMDI) comprising a monomeric MDI content of from 28 wt % to 33 wt % in stoichiometric excess with 2-ethylhexanol or with n-butanol, in the presence of a plasticizer and in the absence of any polyol. The diluent components obtained reached a monomeric isocyanate content as low as 4.3 wt %. In parallel, prepolymer components were prepared from the same polyisocyanate and a polyol, without any mono-alcohol, and which were able to reach a monomeric isocyanate content as low as 3.8 wt %. The prepolymer component having this low MDI content was characterized by an index, as defined elsewhere in this document, of 322. The so obtained prepolymer compositions were blended with the diluent components to form prepolymer compositions in which a monomeric isocyanate content as low as 4.0 wt % was reached. In a comparative example, the same polyisocyanate was simultaneously reacted with 2-ethylhexanol and with a polyol, employing an index of 552, and a higher viscosity prepolymer composition was obtained which contained 7.01 wt % monomeric isocyanate. US 2010/0152381 states in paragraph [0025] that, generally, increasing an amount of the monohydric isocyanate-reactive component relative to the isocyanate component, based on a stoichiometric ratio of OH functional groups to NCO functional groups, decreases the monomeric isocyanate content of the diluent component. US 2010/0152381 however remains silent about how the recently desired very low monomeric isocyanate contents may be achieved in the prepolymer component, or in the prepolymer compositions, or in any further formulation from which a foamed article may be produced.
U.S. Pat. No. 5,880,167 discloses the preparation of prepolymers for hotmelt adhesives, which may also be used for the production of foam plastics from non-reusable pressurized containers. In order to obtain a low content of monomeric volatile isocyanates (essentially diisocyanates), U.S. Pat. No. 5,880,167 proposes to use trifunctional isocyanate monomers, of which the functionality has been reduced by the addition of benzyl alcohol as a terminator. The prepolymers according to this proposal were compared with control examples wherein the trifunctional isocyanate and the benzyl alcohol, together, were replaced by dimethyl methane diisocyanate (MDI) and in which benzyl alcohol was absent. The trifunctional isocyanate monomer could possibly be a mixture containing less than 20 wt % of diisocyanate, based on the weight of the polyisocyanates. The prepolymers based on the trifunctional isocyanate monomers and the benzyl alcohol each time ended up with a significantly lower residual monomeric isocyanate content as compared to their counterparts based on only MDI. U.S. Pat. No. 5,880,167 is silent about any possible effect when benzyl alcohol is combined with polyisocyanates containing at least 30 wt % of diisocyanate.
WO 02/090410 discloses the addition, in the preparation of the prepolymer, of 1-methoxy-2-propanol as a chain terminator, thereby increasing the shelf life of the prepolymer product and lowering the viscosity, and in addition increasing the proportion of open cells in the resulting foam. The amount of monofunctional alcohol used remains rather low, as it represents only 6.12% in equivalents relative to the presence of NCO groups in the prepolymer reaction. The prepolymer may be used in a one component PU foam composition. WO 02/090410 is not concerned with remaining free diisocyanate monomers in the prepolymer or in the foam composition.
U.S. Pat. No. 4,863,994 is concerned with providing reaction injection molded (RIM) parts made from polyurethanes, and proposes the addition of a monohydric alcohol into the reaction blend of the polyols and the polyisocyanate. The document states in column 5, lines 10-17, that the efficacy of the monohydric alcohol in solubilizing all the blend components and to lower blend viscosity will increase with increasing linear molecular conformation, or in simpler terms, with longer molecular distance between the hydroxyl function and the end of the molecule. U.S. Pat. No. 4,863,994 proposes in the examples to use a butyl alcohol initiated polyethyleneoxy-polypropyleneoxy monohydric alcohol with an equivalent weight of 500 in a blend having an isocyanate to active hydrogen ratio of 1.05.
U.S. Pat. No. 5,990,257 discloses various ways to prepare high molecular weight urethane polymers which are endcapped with alkoxyfunctional silanes, i.e. wherein the residual NCO content was brought down to zero and replaced by silane functions. These silylated polyurethanes are able to cure into a siloxane-crosslinked polymer network and are shown to be useful in sealant systems. WO 00/04069 discloses in Example 1 how an alkoxy silane terminated polyurethane prepolymer based on IPDI may be used to provide a suitably performing one component foam (OCF) composition.
US 2010/130674 proposes to react the polyurethane polymer with a compound comprising a group carrying an active hydrogen, such as a hydroxyl group or a mercapto group or a secondary amino group, together with at least one blocked or capped amino group selected from a limited list. The blocked or capped amino group is provided in order to maintain the double functionality when the compound reacts with a diisocyanate monomer, such that the reaction product remains available for incorporation into the high molecular weight polymer which forms during curing. The blocked or capped amino group is, upon exposure to moisture from a substrate or from open air, supposed to at least partially hydrolyze such that the amino group may react with isocyanates and lead to more cross-linking. The additional reagents however are complex, scarce and expensive, and the addition adds significant complexity and cost to the production process and to the resulting product. The method is therefore only economically affordable with prepolymers having a very low NCO value and technically allowable in applications whereby the reaction of NCO with moisture may be slower.
EP 2383304 A1 is also concerned with providing OCF compositions having a low free monomeric MDI content, because of toxicity concern. EP 2383304 proposes to add a significant amount of 2-ethyl hexanol into the composition, compensated with high amounts of triol components, which bring extra cross-linking. At the same time also a significant amount of a bi-functional alcohol having a molecular weight not greater than about 100 g/mol is added as extra hardener, and very high amounts of blowing agents and flame retardant are used, which bring extra dilution. The examples in EP 2383304 also add a significant amount of crude toluene diisocyanate (TDI) (80/20), an isocyanate which is even more volatile and of even higher toxicity concern than MDI, and of which the 2,4-isomer introduces an extremely highly reactive NCO group (i.e. the one located at the 4-position in the benzene ring of the TDI). EP 2383304 A1 states that its exemplified embodiments yield less than 1 wt % free monomeric MDI, but remains silent about any remaining free TDI monomer, which would actually be of even higher concern. The document further provides no analytical results in support of its alleged achievement, nor does it specify what analytical method would be appropriate for determining the content of free monomers. The applicants submit that, in view of the difficulties which need to be overcome in reaching a reliable concentration of less than 1.00 wt % of mMDI, some of which are explained further in this document, EP 2383304 A1 is a non-enabling disclosure because it is not supported by the very specifics about how the mMDI concentrations in the experimental results have to be determined, and an experimental result demonstrating that the alleged result was actually reached.
The applicants hereby report of an analytical test programme in which 6 different samples of PU compositions were analysed by 7 different labs, using methods based on three different analytical principles, i.e. High Performance Liquid Chromatography using a UV detector (HPLC/UV), HPLC coupled with Mass Spectroscopy (HPLC/MS), and GPC using a UV detector. We obtained 9 results for each sample, as some of the labs applied more than one different method. None of the analysis gave more than 1.3 wt % mMDI in any of the samples. The samples were thus very representative for the commercial target of having less than 1.00 wt % mMDI. The test results however showed significant differences between the analytical results. For one sample the results differed from 0.6 wt % up to 1.20 wt %, thus with a factor 2.0, the average being 0.94 wt %. For a second sample the results differed from 0.73 to 1.04 wt %, thus with a factor 1.42, the average being 0.94 wt %. For a third sample the results differed from 0.37 wt % up to 0.91 wt %, thus with a factor as high as 2.46, the average being 0.62 wt %. The applicants conclude that the same sample may give widely different mMDI contents depending on the analytical method and the lab that is chosen for the analysis. The applicants therefore submit that a specification of a particular concentration of mMDI in a PU foam composition is thus non-enabling without a clear specification what the analytical method is that should be used.
WO 2012/101220 A1, and its priority document EP 2481764 A1 which was filed on 27 Jan. 2011, both of which published August 2012, are also concerned with providing OCF compositions having a low free monomeric MDI content. The documents propose to include in the composition, together with crude MDI and 2-ethyl hexanol, a significant amount of an NCO end-capped TDI prepolymer prepared by end-capping with mTDI a polyol having a molecular weight of 3000 that was obtained by reacting an oxypropylene polyether chain with glycol. In example 1 of these publications, the PU foam composition contains 6.76 equivalents % of triols relative to all the isocyanate functions present. In example 2, TDI is present as at least 3.47 wt % relative to all polyisocyanates initially present. The effect of adding the NCO end-capped TDI prepolymer is that the total composition is diluted such that the mMDI concentration is reduced. A drawback is that there may be residual mTDI present in the prepolymer, because the end-capping step requires an excess of mTDI relative to the free alcohol functions present, a monomer which is more volatile than the mMDI which is currently of regulatory concern. The teaching of these documents thus risks to replace the problem of residual mMDI by an even greater problem of residual mTDI. The documents also specify that the NCO end-capped prepolymers of TDI should be prepolymerized prior to their addition to the mixture. This adds an additional step to the preparation of the composition, and hence increases the complexity of the production process. These two documents describe examples which obtain OCF compositions that allegedly yield less than 1 wt. % free monomeric MDI, they however do not specify what analytical method should be used. The applicants submit that therefore the disclosures in these two documents are also non-enabling disclosures.
There therefore remains a need for polyurethane foam formulations having a low residual content of diisocyanate monomer and which also has a viscosity suitable for applying from a pressurized container, and which is obtainable by a process which is simple and low cost.
The present invention aims to obviate or at least mitigate the above described problems and/or to provide improvements generally.